A technique by which transistors are formed using thin semiconductor films formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor including a semiconductor thin film which includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) (an In—Ga—Zn—O-based amorphous oxide) is disclosed (see Patent Document 1). In particular, when the indium content in an oxide semiconductor film is made higher, the mobility of the transistor including the oxide semiconductor film can be increased.